style: rename production flag variable

Renames the current production flag variable to be shorter,
non-Hungarian (presaging changes to come in #35) and not specifically
CSS_-related.

See #80

footnotes.php

@@ -103,4 +103,4 @@ $g_obj_mci_footnotes->run();
  * Developing stylesheets is meant to be easier when this is set to false.
  * WARNING: This facility designed for development must NOT be used in production.
  */
-define( 'C_BOOL_CSS_PRODUCTION_MODE', false );
+define( 'PRODUCTION_ENV', false );